In state-of-the-art pipeline construction, individual lengths of pipes called “joints” are welded together into a continuous length of pipe called a “section.” Pipe joints are taken from a stockpile and loaded onto specialized trucks that haul the joints to a construction site where the joints are off loaded with a side-boom tractor that strings the joints end-to-end along the pipeline right-of-way. Joints measuring up to 80-feet (24.4 meters) long and between about 2 inches (5.08 cm) up to about 72 inches (182.9 cm) in diameter are used in such pipeline construction applications. The joints are usually, but not always, surface coated with a corrosion resistant material for long-term protection.
Prior to offloading the pipe joints, 4″×6″ by 4′-0″ long wood timbers, called skids, are delivered to the jobsite on steel frame structures known as skid baskets. Each skid basket holds approximately 300 wood skids and a tractor-trailer can hold two skid baskets. Alternatively, skids are pallet stacked on a tractor-trailer for delivery and forklift unloading at the job site. A 40-foot tandem-axle tractor-trailer float can carry between 2,000 to 2,500 skids to the jobsite, and a single tractor-trailer float delivery does not normally provide enough skids to complete a one-mile long section. Simultaneous with the laying of the joints, the skids are stacked in alternating layers at spaced apart locations along the pipeline right-of-way to provide a continuous line of skid structures that position the joints at a predetermined work height adjacent a trench. After teams of workers weld the joints together, the assembled section is removed from the skids and buried in the trench. The welded pipeline section is generally lowered into the trench with side-boom tractors. The trench is backfilled and clean-up crews carefully replace the soils. The cleanup crews reload the skids into the skid baskets or onto the tractor-trailer for delivery either to a new location down the line or to a storage area. Such assembly and disassembly of wood skids is labor intensive and costly.
Wood skids are problematic because they are bulky and heavy, making them both awkward to use, and difficult to move to and from construction sites. Their excessive weight causes a variety of lifting and dropping injuries that include strained muscles to broken bones and their tendency to splinter causes a variety of puncture wounds. Additionally, stacked wood is unsatisfactory for use as a pipeline support structure because wood has a high Relative Coefficient of Sliding Friction (RCSF). The high RCSF between the wood skid support structure and coated joints causes the skid support structures to have a propensity for collapsing in response to unexpected movement along lengths of supported sections. For example, it is not uncommon for a worker to bump a section with a vehicle or other piece of equipment during the construction phase. Such impacts, even when minor, tend to generate a lateral force along the partially completed section, and the high RCSF between the wood skid support structure and pipe surface causes supported sections to collapse. Such a section collapse results in extensive damage to the pipe and to its protective corrosion coating. A section collapse is also a major cause of injuries and fatalities at a pipeline construction site. Proper repair of damaged joints along a collapsed section requires numerous man-hours of intensive labor to reassemble collapsed wood skids, elevate the fallen section, and repair any damaged to the joints and/or the corrosion coating.
Another problem associated with wood skids, as well as with other high friction materials used for pipeline support structures, is related to ambient temperature changes at the construction site. Fluctuating temperatures alternately heat and cool the supported section. This causes expansion and contraction along the section and generates a lateral force against the skid support structures. Such lateral forces can cause the skid support structures to collapse when the pipe section is not able to slide freely along the contacting surfaces.